In the wet type developing apparatus, an anilox roller (feed roller) immersed in part in a liquid developer is driven to rotate in rotational contact with a developing roller with which the photoconductor drum is driven to rotate in rotational contact, and the liquid developer is fed onto the developing roller via the anilox roller. With the developing roller driven to rotate in rotational contact with a photoconductor drum, an electrostatic latent image formed on the photoconductor drum is developed into a toner image. And, the toner image developed on the photoconductor drum is transferred onto a web of paper (recording medium).
In a conventional wet type developing apparatus of this sort, by voltage application from a power supply to the anilox roller, a voltage becoming a developing bias is applied to the developing roller. And, positive voltages as the bias voltages are applied from to the anilox roller and the developing roller. While the voltage is being controlled aptly, a thin film made of a uniform toner layer is formed on the developing roller and, with the toner of this thin film an electrostatic latent image on the photoconductor drum is developed in printing on a web of paper (see JP 2009-192920 A). The film thickness of the toner layer then on the surface of the developing roller exerts large influences on the print density and stability in print quality.
In the conventional wet type developing apparatus mentioned above, when the film thickness of the toner layer formed on the surface of the developing roller is controlled by changing the roller bias applied to the anilox roller, a small difference in electric potential between the developing roller the anilox roller to which the roller bias is applied gives rise to the problem that as the printing speed is increased or due to changes in physical property of the liquid developer, such inconveniences as a drop in image density, a deterioration in reproducibility of halftone dots, occurrence of streaks and unevenness and appearance of a trace of a cell of the surface of the anilox roller on the surface of an image, tend to be brought about, making it difficult to control so as to ensure print quality such as of print density, reproducibility of images and stability of the print quality in continuous printing.
Also, in order to keep the print quality from deteriorating due to such as a drop in density, there has been proposed an arrangement as shown in FIG. 5 of JP 2007-147977 A in which a compaction roller is driven to rotate in rotational contact with the periphery of the developing roller to compact the liquid developer (or to make it in the form of a thin film thereof) on the developing roller and further a cleaning roller is driven to rotate in rotational contact with the developer roller to effect cleaning of the developing roller. This arrangement has the problem, however, that mounting such compaction and cleaning rollers becomes a hindrance when the developing roller is exchanged, thereby worsening the apparatus maintainability.
When the compaction roller is used, the problem also arises that compaction tends to be uneven due to such as roller eccentricity, making it hard for the liquid developer to be filmed uniformly.
Also, in developing the latent image on the surface of the photoconductor drum driven to rotate in rotational contact with the developing roller, it is seen that the thinner the film thickness of the toner layer on the surface of the developing roller, the less the amount of the liquid developer that can be used and the more uniform the thickness of the thin film that can be made. Also, the lower the viscosity of the liquid developer, the more facile its toner agitation, and hence the lower the cost at which it can be manufactured, rendering it less expensive. In general, however, when the viscosity of the liquid developer is 100 mPa·s or less, the lower the toner density, the less dispersible the toner particles. From the fact, therefore, that if the film thickness of the toner layer formed on the surface of the developing roller is not more than 5 μm, the print density tends to be deficient, conventionally a toner layer having film thickness of 5-40 μm has been used for development, resulting in a large amount of use of the liquid developer.
On the other hand, in consideration of the above, the conventional use of a liquid developer that has a viscosity as high as 100-1000 mPa·s has presented the problem that it becomes difficult to agitate the toner, rendering its cost of manufacture in the apparatus exceedingly high.
The present invention has been made to solve the abovementioned problems in the prior art. It is a first object of the present invention to provide a wet type developing apparatus for use in a wet type electrophotographic printer which allows printing in a stable quality at an increased printing speed and also with changes in physical property of a liquid developer, by making it possible to increase the difference in electric potential of toner particles on the surface of an anilox roller relative to a developing roller.
It is a second object of the present invention to provide a wet type developing apparatus which allows using a liquid developer low in viscosity and making thinner the film thickness of a toner layer formed on a surface of the developing roller to reduce the cost of the liquid developer, which with the film thickness on the surface of the developing roller made thinner, additionally allows forming the toner layer of a uniform thickness and reducing the amount of use of the liquid developer, which by increasing the difference in electric potential of toner particles on the surface of the developing roller, further allows developing an electrostatic latent image of a photoconductor roller at a high density to rise the high printing speediness by preventing the print quality from deteriorating due to such as a drop in density, which can also facilitate cleaning of residual toner on the surface of the developing roller to prevent the toner from caking on the developing roller, which also allows effecting maintenance of the developing and anilox rollers without hindrance by a cleaning roller, which further allows maintaining proper and constant the contact pressure of a cleaning blade in contact with the surface of the anilox, the developing or the cleaning roller simply by spring urging, and which additionally allows preventing the developing roller from being flexurally deformed.
It is a third object of the present invention to provide a wet type developing method which allows preventing the developing roller from flexural deformation by avoiding the developing roller and the photoconductor drum from contacting with each other in the state that they cease rotating and which allows conditioning rotation of the anilox and developing rollers before the developing roller comes into rotational contact with the photoconductor drum to maintain constant and uniform the density of an image being developed in a developing operation after the developing roller comes into rotational contact with the photoconductor drum.